BoilerFaultCodesThe 6 most common faults
Your boiler has shut down because the internal water temperature has become too hot, likely due to a lack of water flow or a circulation problem.
Why it happens: This fault occurs when the water inside the boiler heats up too quickly or reaches an unsafe temperature, forcing the system to shut down for safety. The most common reason is a failure in the circulation pump or a blockage in the pipes, which prevents the hot water from moving away from the burner. It can also be caused by a faulty sensor that is incorrectly reporting the water temperature to the control board.
Check if all radiator valves are open to ensure water can flow.Check the boiler pressure gauge and top up to 1.5 bar if it is too low.Restart the boiler using the reset button to see if the fault clears.
Your boiler is failing to light the gas burner or is losing the flame while it is running, meaning you will have no heating or hot water.
Why it happens: This fault occurs when the boiler's internal computer tries to light the gas burner but cannot confirm that a steady flame is present. The most common reasons are a blocked or dirty ignition probe, or a problem with the gas supply preventing the burner from catching light correctly.
Check if your gas supply is active by testing another gas appliance like a hob.If the weather is freezing, check if the white plastic condensate pipe leading outside is frozen and thaw with warm water.Press and hold the 'Reset' button for five seconds to see if the boiler restarts.
Your boiler has lost its connection to the main electrical supply or has detected a significant power interruption.
Why it happens: This fault occurs when the boiler's main control board detects a sudden loss or interruption in the electrical supply. It is often triggered by a localized power cut, a blown fuse within the unit, or a loose internal wiring connection that breaks the circuit. In some cases, a failing electronic component on the main circuit board can no longer process the incoming power correctly.
Check if there is a general power cut in your streetInspect your home's consumer unit (fuse box) to see if a circuit breaker has trippedEnsure the boiler's isolation switch (fused spur) is turned onPress the Reset button on the boiler control panel
Your boiler's internal pump is either jammed or struggling to move water because there is too much trapped air in the system.
Why it happens: This fault occurs when the boiler's internal pump is unable to circulate water because it is physically jammed or there is a large pocket of air preventing it from gaining traction. Over time, debris in the system water can seize the pump's moving parts, or air can enter the system after a period of inactivity or a recent repair.
Check your boiler pressure gauge and top it up to 1.5 bar if it is too lowBleed all of your radiators to remove trapped airGently restart the boiler using the reset button
Your boiler is overheating because water is not moving through the system quickly enough to carry heat away.
Why it happens: This fault occurs when the water inside the boiler heats up too quickly because it isn't circulating through your radiators or pipes as it should. The most common reasons are a failed internal pump that has stopped spinning, or a blockage caused by a buildup of sludge and debris within the main heat exchanger.
Check that all radiator valves are fully openCheck that the boiler pressure is between 1 and 1.5 bar on the pressure gaugeBleed all radiators to remove trapped air that might be blocking water flow
Your boiler is shutting down because it is heating up much faster than it should, usually due to water not flowing through the system properly.
Why it happens: This fault occurs when the water inside the boiler heats up too quickly because it isn't being moved away into your radiators or hot taps fast enough. The most common reasons are a pump that has stopped working, a blockage of sludge or limescale in the pipework, or air trapped within the system.
Check that your radiator valves are fully openEnsure the central heating system pressure is between 1.0 and 1.5 barBleed all radiators to remove trapped air that might be blocking flow
on the Worcester Bosch Greenstar Si Combi / i System
Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.
Call a Gas Safe engineer if…
- You can smell gas or see signs of a leak
- The Worcester Bosch shows an Emergency or High severity code
- The boiler keeps locking out after repeated resets
- You've tried the DIY checks and the fault hasn't cleared
- There's visible water leaking from the boiler
- The flame is yellow or orange instead of blue